For example, in a bearing device for supporting the swing arm of a magnetic disk device (hard disk drive), radial stiffness or rigidity of the bearing device has an important effect on performance.
Heretofore, there is disclosed a management technique for a bearing device involving managing the clamping torque of each part in the bearing device, the weight of each part, and fit tolerance and the like, while measuring the radial resonance frequency of the bearing device, to thereby keep the radial resonance frequency of the bearing device within a predetermined range which is previously set (refer for example to Japanese Patent Publication No. Tokukai Hei 2001-83045 (pages 3, 4, and FIG. 1)
When measuring the resonant frequency of a bearing device, an additional mass member is fixed to the bearing device to measure the resonant frequency, so that the resonant frequency due to radial stiffness or rigidity is reduced, with the amplitude of the resonance peak increased, thus simplifying detection of the resonant frequency.
Furthermore, the additional mass member fixed to the bearing device increases the inertial moment so as to effectively lower the resonant frequency of the conical mode of the bearing device, and to enable an increase in the difference with the resonant frequency of the radial translation mode, thus enabling more accurate measurement of the resonant frequency.
When the bearing device is incorporated within another device (for example a magnetic disk device), the mass of the additional mass member may be made equivalent to the mass of the load on the bearing device. As a result, the resonant frequency during measurement may be brought closer to the actual resonant frequency, so that for example the performance of the magnetic disk device is improved.
As shown in FIG. 1(a), a bearing device 2 comprises a double row or pair of bearings 6 each having an outer ring 4 and an inner ring 5, a shaft 7 fitted through both of the inner rings 5, and a housing 3 fitting over both of the outer rings 4, with the shaft 7 and housing 3 being able to rotate freely relative to each other
An additional mass member 1 according to a conventional example for measuring the resonance frequency of the bearing device is fixed to the housing 3 arranged on the bearing device 2. Specifically, the conventional additional mass member 1 is formed with a tapered inner periphery la and fixed to the housing 3, as shown in FIGS. 1(a), by fitting the tapered inner periphery 1a onto the housing 3.
An additional mass member 10 according to another conventional example shown in FIG. 1(b) has a straight-shaped inner periphery 10a, and is fitted onto the housing 3 and then fixed to the housing 3 with a screw 11.
Alternatively, an additional mass member according to another conventional example may be bonded to the housing and thus fixed in place.
In practice, as the device such as a magnetic disk device (hard disk drive) or the like in which the bearing device is incorporated is miniaturized while its speed increased, the need for reduced size and weight, and greater stiffness or rigidity, of the bearing device is increased.
In order to reduce size and weight, a bearing device has been developed such that no housing is used in the bearing device, and that the outer rings are fixed with a spacer mounted between the outer rings. Furthermore, an appropriate preload is applied to the bearing device in order to increase stiffness or rigidity.
In the case of the bearing device formed with the spacer mounted between the outer rings, when the tapered inner periphery of the additional mass member is fitted on the outer rings, for fixture by conventional means, the additional mass member is fixed to only one bearing of a double row or pair of bearings When a bearing device wherein the additional mass member is fixed to only one bearing, is mounted on the resonance measuring apparatus and then the resonant frequency measured, it is difficult to obtain the resonant frequency based on the sum of the stiffness or rigidity of the double row or pair of bearings, so that there is a problem in that it becomes difficult to exhibit the expected predetermined performance of the bearing device.
Moreover, according to the conventional methods of fixing the additional mass member, when a screw is used for fixing, the tip of the screw may damage the bearing device, while when bonding is used the bonding agent is difficult to remove, thus making attachment and removal of the additional mass member to the bearing device difficult, leaving room for improvement.